The present invention relates to a metal vapor discharge lamp having a starting device of a thermal switch type.
Inside a metal vapor discharge lamp having an arc tube or burner such as a high pressure sodium lamp, mercury Hg and sodium are sealed together with an inert gas for starting. As Xenon (Xe) has a small thermal conduction loss and is capable of increasing luminous efficacy, it is generally used as an inert gas for starting. However, unlike the case where argon (Ar) is used, a Penning effect does not occur between Hg and Xe. As a result, the starting voltage of the burner is increased, and the burner cannot start with a commercial power supply voltage. In order to turn on a high pressure sodium lamp of this type, a ballast, having an expensive starting device for generating a high voltage pulse, is required.
On the other hand, a high pressure sodium lamp that can be started by even an inexpensive mercury lamp ballast has recently been developed. This lamp comprises: a burner; a starting device of the thermal switch type, which is constituted by a series circuit of a thermal switch and a coil filament and which is connected in parallel with the burner; and an outer envelope for hermetically sealing the burner and starting device inside the lamp. An outside auxiliary conductor is disposed in contact with a tube wall of the burner to decrease the required starting voltage. In a lamp of this type, if the burner develops a leak toward the end of its service life, xenon, mercury and sodium will leak out of the burner into the outer envelope, which maintains a high vacuum. When the power switch is turned on and power is supplied to the lamp while a gas pressure of xenon, leaking into the outer envelope, is held to be not less than 13.3 Pa (0.1 torr), the starting device is actuated to generate a pulse, and the burner may be turned on. In this case, the large amount of mercury sealed in the burner leaks into the outer envelope, and the lamp voltage will not be increased. Therefore, a current similar to a short circuit current flowing through the secondary side of the ballast flows through the ballast, possibly overheating the ballast.
Japanese Laid Open Publication No. 55-122351 discloses a means for causing a discharge in the outer envelope to melt and disconnect a coil filament so that a high voltage pulse, generated from the starting device while there is leakage from the burner, will not be applied to the ballast to break down the ballast. The coil filament of the starting device of a thermal switch type is held at a temperature of 1873 K. (1,600.degree. C.) or higher during its operation. If the burner leaks, xenon in the outer envelope is ionized by the thermo electrons produced from the coil filament, so that a discharge occurs in the outer envelope with the coil filament acting as an electrode. By this discharge, the coil filament is melted and disconnected.
However, according to the prior art described above, the coil filament can be melted and disconnected only when the xenon pressure in the outer envelope exceeds 26.6 Pa (0.2 torr). As described above, the burner tends to be turned on in accordance with a kick voltage generated upon operation of the thermal switch even if the xenon pressure in the outer envelope is less than 26.6 Pa. In general, the on/off operation of the thermal switch can be performed only once every 2 to 3 seconds. When the lamp is installed at a place (e.g., in the vicinity of mechanical equipment or traffic) where vibration occurs, the contact of the thermal switch tends to vibrate and turn the switch on/off about 10 times per second. In this condition, the lamp is turned on, and the coil filament will neither be melted nor disconnected even if the gas pressure of xenon leaking into the outer envelope has not reached 26.6 Pa. As a result, the ballast is damaged due to overheating.
Another problem of the conventional lamp lies in the fact that it takes a long time to melt and disconnect a coil filament even after the temperature of the coil filament exceeds 1,600.degree. C. In this case, the thermal switch tends to be turned on before the coil filament reaches a temperature high enough for it to be melted and disconnected. When the gas pressure of xenon, leaking into the outer envelope, exceeds 13.3 Pa, the lamp is turned on. As a result, the damaging of the ballast due to overheating cannot be completely prevented.